Mayor de Blasio is creating a new “safe space” for city students — by installing single-stall bathrooms in every public school. Still missing is any real word on how much it will cost — and some explanation of how it makes sense.

After all, school lavatories have been open to all gender identities since 2014, with no one required to use a bathroom that “conflicts” with his or her identity.

De Blasio reaffirmed that policy last year: “No one deserves to be denied access to bathrooms or discriminated against for being who they are.”

Chancellor Carmen Fariña says the new policy (which also includes disabled kids and those with other medical conditions) provides privacy without forcing students to produce documentation or get permission.

No one should face harassment or embarrassment, in the lavatory or anywhere else. But de Blasio and Fariña haven’t alerted the public to any ongoing harassment problem.

And if kids are being harassed, new bathrooms won’t stop it: School leaders have to act.

The conversions start this month, with $1 million worth of signage and other changes to convert existing single-stall bathrooms for student use. Officials have no estimate of the final price tag.

All that’s certain is that Team de Blasio is checking off another progressive box, spending money to solve a problem it has suggested no longer exists.